Sections of UV unwrap not taking texture painting
If your object is a single object made up of multiple meshes, and you are adding materials to these meshes, then you need to make sure the materials are all sharing the image texture that you are painting for them to show the effect the texture is meant to describe.

A color image meant to give color to the model has to be shared for all the materials to be included with the paint - and even if you aren't going to use color on several mesh parts, those materials might be sharing a single image for bumps/gloss/etc. 

In the actual case above, OP needs to make sure the model in question isn't composed of multiple objects and is indeed one object - and that the materials added all share the same image texture.